1. Field of the Invention
The present invention relates to a power conversion circuit for driving fluorescent lamps in a backlight system, and more particularly relates to a lamp inverter for improved detection of a shorted lamp condition in the backlight system.
2. Description of the Related Art
Fluorescent lamps are used in a number of applications where light is required but the power required to generate the light is limited. One particular type of fluorescent lamp is a cold cathode fluorescent lamp (CCFL). CCFLs are used for back lighting or edge lighting of liquid crystal displays (LCDs). LCDs are typically used in notebook computers, web browsers, automotive and industrial instrumentations, and entertainment systems. Each LCD typically uses multiple CCFLs.
CCFL tubes typically contain a gas, such as argon, xenon, or the like, along with a small amount of mercury. After an initial ignition stage and the formation of plasma, current flows through the tube. The current causes the generation of ultraviolet light. The ultraviolet light strikes a phosphorescent material that coats the inner wall of the tube to cause the phosphorescent material to emit visible light.
A power conversion circuit (e.g., an inverter) is generally used for driving one or more CCFLs. The power conversion circuit accepts a direct current (DC) input voltage and provides an alternating current (AC) output voltage to the CCFLs. The brightness (or the light intensity) of the CCFLs is controlled by controlling the current (i.e., the lamp current) through the CCFLs. For example, the CCFLs can be dimmed or brightened by decreasing or increasing the average lamp current.
CCFLs are susceptible to defects or damage, which can cause short circuit conditions that may damage the power conversion circuit. The power conversion circuit is typically difficult and expensive to replace after installation. Thus, shorted lamp protection is generally provided to protect the power conversion circuit during a shorted lamp condition. The impedance of an operable CCFL is typically between 80 kilohms and 100 kilohms. The shorted lamp condition occurs when the impedance across the CCFL is significantly lower (e.g., less than 2 kilohms). This shorted lamp condition is typically detected by sensing the lamp current. For example, a sensing transformer or a sensing resistor can be coupled in series with the CCFLs to sense the lamp current and to provide a feedback signal to the power conversion circuit. The power conversion circuit may shut down when the average lamp current becomes excessive, which indicates a shorted lamp condition.
One problem with sensing the lamp current to detect the shorted lamp condition is that some shorted lamp conditions may not be reliably detected, especially when the power conversion circuit drives multiple CCFLs. For example, the lamp current may only increase 20%-30% when one CCFL is shorted in a multiple CCFL configuration. The 20%-30% increase may be within the range of operating lamp currents for increasing the intensity of the CCFLs and may not trigger the shorted lamp protection. Furthermore, the sensing transformer used in some applications has a current limit which can impede the detection of the shorted lamp condition. In addition, lamp current sensing does not sense a short circuit condition at the output of the power conversion circuit, which can be caused by improper installation of the power conversion circuit or the CCFLs.